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The following page has been changed by DougCutting:
http://wiki.apache.org/hadoop/HadoopMapReduce
The comment on the change is:
update for TLP move
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== Map ==
As the Map operation is parallelized the input file set is first
- split to several pieces called [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/FileSplit.html
FileSplits]. If an individual file
+ split to several pieces called [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/FileSplit.html
FileSplits]. If an individual file
is so large that it will affect seek time it will be split to
several Splits. The splitting does not know anything about the
input file's internal logical structure, for example
@@ -18, +18 @@
When an individual map task starts it will open a new output
writer per configured reduce task. It will then proceed to read
- its !FileSplit using the [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/RecordReader.html
RecordReader] it gets from the specified
+ its !FileSplit using the [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/RecordReader.html
RecordReader] it gets from the specified
- [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/InputFormat.html InputFormat].
!InputFormat parses the input and generates
+ [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/InputFormat.html
InputFormat]. !InputFormat parses the input and generates
- key-value pairs. !InputFormat must also handle records that may be split on the !FileSplit
boundary. For example [http://svn.apache.org/viewcvs.cgi/lucene/hadoop/trunk/src/java/org/apache/hadoop/mapred/TextInputFormat.java?view=markup
TextInputFormat] will read the last line of the !FileSplit past the split boundary and, when
reading other than the first !FileSplit, !TextInputFormat ignores the content up to the first
newline.
+ key-value pairs. !InputFormat must also handle records that may be split on the !FileSplit
boundary. For example [http://svn.apache.org/viewcvs.cgi/hadoop/core/trunk/src/java/org/apache/hadoop/mapred/TextInputFormat.java?view=markup
TextInputFormat] will read the last line of the !FileSplit past the split boundary and, when
reading other than the first !FileSplit, !TextInputFormat ignores the content up to the first
newline.
It is not necessary for the !InputFormat to
generate both meaningful keys ''and'' values. For example the
@@ -30, +30 @@
offsets.
As key-value pairs are read from the !RecordReader they are
- passed to the configured [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/Mapper.html
Mapper]. The user supplied Mapper does
+ passed to the configured [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/Mapper.html
Mapper]. The user supplied Mapper does
- whatever it wants with the input pair and calls [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/OutputCollector.html#collect(org.apache.hadoop.io.WritableComparable,%20org.apache.hadoop.io.Writable)
OutputCollector.collect] with key-value pairs of its own choosing. The output it
+ whatever it wants with the input pair and calls [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/OutputCollector.html#collect(org.apache.hadoop.io.WritableComparable,%20org.apache.hadoop.io.Writable)
OutputCollector.collect] with key-value pairs of its own choosing. The output it
generates must use one key class and one value class. This is because
- the Map output will be written into a [http://wiki.apache.org/lucene-hadoop/SequenceFile
SequenceFile]
+ the Map output will be written into a SequenceFile
which has per-file type information and all the records must
have the same type (use subclassing if you want to output
different data structures). The Map input and output key-value
@@ -41, +41 @@
When Mapper output is collected it is partitioned, which means
that it will be written to the output specified by the
- [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/Partitioner.html Partitioner].
The default [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/lib/HashPartitioner.html
HashPartitioner] uses the
- hashcode function on the key's class (which means that this hashcode function must be good
in order to achieve an even workload across the reduce tasks). See [http://svn.apache.org/viewcvs.cgi/lucene/hadoop/trunk/src/java/org/apache/hadoop/mapred/MapTask.java?view=markup
MapTask] for details.
+ [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/Partitioner.html
Partitioner]. The default [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/lib/HashPartitioner.html
HashPartitioner] uses the
+ hashcode function on the key's class (which means that this hashcode function must be good
in order to achieve an even workload across the reduce tasks). See [http://svn.apache.org/viewcvs.cgi/hadoop/core/trunk/src/java/org/apache/hadoop/mapred/MapTask.java?view=markup
MapTask] for details.
N input files will generate M map tasks to be run and each map
task will generate as many output files as there are reduce
@@ -80, +80 @@
== Reduce ==
When a reduce task starts, its input is scattered in many files across all the nodes where
map tasks ran. If run in
distributed mode these need to be first copied to the local
- filesystem in a ''copy phase'' (see [http://svn.apache.org/viewcvs.cgi/lucene/hadoop/trunk/src/java/org/apache/hadoop/mapred/ReduceTaskRunner.java?view=markup
ReduceTaskRunner]).
+ filesystem in a ''copy phase'' (see [http://svn.apache.org/viewcvs.cgi/hadoop/core/trunk/src/java/org/apache/hadoop/mapred/ReduceTaskRunner.java?view=markup
ReduceTaskRunner]).
Once all the data is available locally it is appended to one
file in an ''append phase''. The file is then merge sorted so that the key-value pairs for
a given key are contiguous (''sort phase''). This makes the actual reduce operation simple:
the file is
read sequentially and the values are passed to the reduce method
with an iterator reading the input file until the next key
- value is encountered. See [http://svn.apache.org/viewcvs.cgi/lucene/hadoop/trunk/src/java/org/apache/hadoop/mapred/ReduceTask.java?view=markup
ReduceTask] for details.
+ value is encountered. See [http://svn.apache.org/viewcvs.cgi/hadoop/core/trunk/src/java/org/apache/hadoop/mapred/ReduceTask.java?view=markup
ReduceTask] for details.
At the end, the output will consist of one output file per executed reduce
task. The format of the files can be specified with
- [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/JobConf.html#setOutputFormat(java.lang.Class)
JobConf.setOutputFormat]. If !SequentialOutputFormat is used then the output key and value
+ [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/JobConf.html#setOutputFormat(java.lang.Class)
JobConf.setOutputFormat]. If !SequentialOutputFormat is used then the output key and value
classes must also be specified.